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A cure for the plague of frogs?

One of the worst scourges of frogs and their kin is Batrachochytrium dendrobatidis (Bd), a deadly fungus that infects nearly half of amphibian species, eats away their skin, and causes heart attacks. Now, a study shows that one kind of frog can learn to avoid the widespread fungus and that two species become resistant with repeated exposures. Although preliminary, the findings suggest that there may be a way to help protect more vulnerable amphibians.

“If frogs exposed and cured are therefore ‘vaccinated’ against Bd, then this study has big implications for mitigation of Bd outbreaks in the future and restoration of frog populations that are in captive holding now,” writes biologist Kelly Zamudio of Cornell University, who was not involved in the study, in an e-mail.

Amphibians suffer from many threats, including pollution and habitat destruction. It isn’t easy to pin an extinction on Bd, but researchers suspect the fungus is behind the mysterious decline of many species.

To study how well frogs can resist the fungus, Jason Rohr, an ecologist at the University of South Florida, Tampa, and colleagues set up experiments with two species that are easy to catch in Florida, the oak toad (Bufo quercicus) and the Cuban tree frog (Osteopilus septentrionalis).

First, they investigated whether oak toads could learn to avoid the fungus. After setting up a small chamber in their laboratory, they added the fungus to one side. When oak toads were first put into the chamber, they spent equal amount of time on either side. Then, Rohr and his colleagues removed the toads and killed the fungus with moderate heat. When the same toads were placed back into the chamber, they were less likely to spend time on the side with the fungus, suggesting that they can learn to avoid it. Rohr has additional experiments under way to determine how the toads detect the fungus, but he suspects that contact is painful as the fungus attacks. “It can’t feel good to have your skin digested by an enzyme.”

Other trials with both species showed that with each exposure to the fungus, the toad’s and frog’s immune systems strengthened their response. The animals roughly doubled the number of immune cells by the fourth time they encountered the fungus. And a greater proportion of the amphibians survived: Although only 20% made it through the first exposure, more than half escaped their fourth bout with the fungus, the team reports in this week’s issue of Nature. This finding isn’t relevant to oak toads and Cuban tree frogs in the wild, because the weather is hot enough where they live to usually kill the fungus. But it could help explain how some species in colder environments, such as mountain lakes, have managed to survive.

An intriguing finding is that exposure to dead Bd can also increase resistance, which raises the prospect of immunizing highly sensitive species. “That is exciting stuff,” says biologist Karen Lips of the University of Maryland, College Park, although she describes the level of protection as moderate. Rohr hopes to try adding large amounts of dead Bd to ponds to test whether it will help the survival of susceptible frogs.

Several important questions remain. It’s not clear whether the resistance is permanent, whether tadpoles can become resistant, or whether it occurs in other species and would be strong enough to save them. And Erica Bree Rosenblum of the University of California, Berkeley, points to logistical challenges of trying to use the discovery, such as through widespread treatment of endangered populations. “I tend to be skeptical about whether it will be easy to translate results from lab studies into direct conservation application.”